Intracranial pressure (ICP) measurement is an extremely
important part of the neurosurgical armamentarium. The raised ICP the is not
only the commonest cause of death in neurosurgical patients, it is extremely
common in patients suffering from head injury. The effective treatment of
raise ICP has been shown to decrease mortality. Obviously, an understanding of
the principles of ICP measurement is an important prerequisite consideration
to the disturbances of brain function that follow head injury. ICP monitoring
has been used in subarachnoid hemorrhage, hydrocephalus, brain tumors,
infarctions, non traumatic intracerebral hemorrhage, prognostication and
treatment, but the most prominent use is in the field of head trauma. Since
the preponderance of available literature deals with its use in trauma, the
greater part of this review will inevitably deal with head injury.

Intracranial pressure (ICP) has been systematically
measured only from the last half of a century, but the concept of raised
pressure has been known for centuries, and was measured metrically by Quincke
in 1897.

1
The seminal publication on ICP monitoring was by Guillame and Janny in 1951.
This study unfortunately did not gain the publicity it deserved as it was
published only in French. The first widely read paper on systematic ICP
monitoring was by Lundberg in 1960, in which he acknowledged Janny’s earlier
work. Subsequently there have been numerous important publications on the
incidence, pathophysiology and influence of raised ICP on outcome from various
intracranial pathologies, but the next major impetus towards increasing the
incidence of routine ICP monitoring was the publication of the Brain Trauma
Foundation guidelines in 1995 and their updates in 2000.2
ICP monitoring has been used in subarachnoid hemorrhage, hydrocephalus, brain
tumors, infarctions, nontraumatic intracerebral hemorrhage, prognostication
and treatment, but the most prominent use is in the field of head trauma.
Since the preponderance of available literature deals with its use in trauma,
the greater part of this review will inevitably deal with head injury.

Patho-physiology: The fundamental Munro Kelly concept
is that the intracranial cavity is a closed and rigid compartment with three
components: They are brain (80%), blood (12%) and CSF (8%). Its total volume
is 1600_ml, and that increase in any one of these components can be achieved
only at the expense of another. Thus in the event of a growing mass lesion in
the brain, the initial response would be a decrease in the volume of CSF and
blood (mainly from the venous sinuses), and once this compensatory mechanism
failed, the ICP would begin to rise significantly. This is the principle that
underlies all the causes of raised ICP, most of which are multifactorial. The
quest for developing the ideal method of recording ICP has been a difficult
exercise. The first requirement for any method is that it is accurate; and it
should also be safe and simple.

3
The search is still incomplete, since all current methods are invasive. The
necessity to breach the skull to record ICP has resulted in a significant
number of neurosurgeons being reluctant to embrace this technique. It took at
least 15 years before ICP monitoring became fully accepted into clinical
neurosurgical practice in more than a few centers. Even now, opinions vary as
to the value of the technique, from those who claim that it makes no
difference to the outcome of any neurosurgical disease to those who assert
that it is an indispensable part of neurosurgical practice, without which many
patients would surely die. The truth lies somewhere between these two extremes
and it depends on the facilities and personnel available in any given
neurosurgical unit.4

Measurement of ICP

Historical aspects

1. Lumbar Puncture:

Lumbar puncture was introduced into clinical medicine in 18975
and following this, the spinal
CSF pressure was used as an indirect measure of ICP. Sharpe published a
monograph on head injury in 1920 and stated that his principal indication for
the operation of sub temporal decompression was a spinal fluid pressure above
15mmHg.6
Jackson also advocated the use of lumbar puncture and pressure measurement in
head injury in 1922,7
but there was much disagreement on the place and dangers of lumbar puncture,
and the reliability of the procedure in accuratelymeasuring ICP. The two
principal objections to lumbar puncture in the diagnosis of intracranial
hypertension have been the danger of inducing brain-stem compression through
tentorial or tonsillar herniation and the contention that spinal fluid
pressure is not always an accurate reflection of ICP. Langfitt’s work was
particularly important in demonstrating this lack of correlation between ICP
and spinal CSF pressure under conditions of high ICP.8

2. Ventricular puncture: Ventricular puncture for the
relief of increased ICP is one of the oldest practices in neurosurgery.
Pressure measurements during this procedure were often done but prolonged
pressure measurements were performed infrequently because water and mercury
manometers were cumbersome and also because of the risk of intracranial
infection.

Current aspect: The development of strain gauges
allowed ICP measurement to be performed directly using a ventricular catheter
and an external transducer. The pioneering neurosurgeons in its development
were Janny and Lundberg (1960). Since then, the technique has been widely
adopted, with some variations. There are other several techniques available
for monitoring that vary in accuracy, ease of use and cost. These have been
ranked by the Brain Trauma Foundation

2
based on their accuracy, stability and ability to drain CSF as follows:

Transducers: Transducers for measuring pressure are
based on strain gauges, which were originally developed by Engineers and were
able to measure the effects of tension and compression in beams. The applied
force (per unit area) is called the stress and the resulting increase in
length (per unit length) is called the strain. The operation of a wire strain
gauge depends on the fact that if a length of wire is stretched, its
electrical resistance will increase, and vice versa.

In a commonly used transducer such as the Statham P23
series, four strain-sensitive wires are connected to two frames, one of which
fits inside the other. The wire frame is attached to the diaphragm of the
transducer (upon which the pressure acts) and the outer frame is fixed. The
set of wires form a Wheatstone bridge network, which is energized by a direct
current. A stable DC amplifier is used to detect the imbalance of the DC
bridge, and this signal can then be used to drive a pen recorder or be
displayed on an oscilloscope. It goes without saying that the staff in an
intensive care unit must be familiar with calibrating and maintaining pen
recorders.

Catheter-tip transducer: Catheter-tip transducers have
been used for several years and this is currently the preferred method of
recording ICP. Miniature implantable transducers have been developed from
intravascular transducers, of which the Camino transducer is an example.
Pressure is measured at the tip of a narrow fiberoptic catheter where there is
a flexible diaphragm. Light is reflected off the diaphragm and these changes
in light intensity are interpreted in erms of pressure. The outside diameter
of the device is only 4 FG (1.3mm). The system is not dependent on a fluid
column, or on an external transducer where the height needs constant
readjustment depending on the level of the patient’s head. Ostrup and
Crutchfield have reported excellent results but cost is still a problem.

9,10
There is a close correlation
between ICP measured by the Camino catheter-tip transducer and the
intraventricular method.11 The Inner space transducer is a similar
type of a fiberoptic catheter-tip transducer, but the physical principle uses
spectral frequency. Marmarou has reported both experimental tests on this
transducer.12,13 The main limitation of a catheter-tip
transducer is that it is not possible to calibrate it in situ and it should be
replaced if monitoring is to be continued for longer than 5 days, because of
possible drift. They are simple to insert and we place the tip in the brain at
a depth of 1–2cm. The fiberoptic cable can be damaged by restless patients or
if it is bent acutely, and this fragility is a practical problem and is one
that limits the usefulness of the method.

Implanted microchip transducer: Implanted microchip
sensors have now been developed and an example is the Codman Micro Sensor
transducer. It consists of a miniature solid state pressure sensor mounted in
a very small titanium case (diameter 1.2 mm = 3.6 FG) at the tip of a 100 cm
long flexible nylon tube (diameter 0.7mm 2.1FG). The transducer tip contains a
silicon microchip with diffused piezoresistive strain gauges which are
connected by wires in the nylon tube to complete a Wheatstone bridge type
circuit. When the transducer is energized and pressure is applied, the silicon
diaphragm deflects a small amount (less than 0.001mm3 for 100mmHg applied
pressure), inducing strain in the embedded piezoresistors. This resistance
change is reflected in the form of a differential voltage which is then
converted into units of pressure, i.e millimeters of mercury. The bottom layer
of the silicon diaphragm is vented to the atmosphere along the nylon tube,
while the top layer is exposed to the applied CSF or brain tissue pressure.
The microsensor transducer can be inserted directly into the brain parenchyma
but is also fine enough to be passed through a catheter into the lateral
ventricle. Narayan and his colleagues found that this device had an average
drift of less than 1mmHg over a 9-day period.

14
This group also tested the Codman transducer in 25 patients, comparing it
against a ventricular catheter and an external transducer.15
Encouragingly low levels of baseline drift were found and it showed no
tendency to under-read or over-read. Piper and Miller evaluated the wave-form
analysis capability of this transducer against a fluid coupled transducer.16
They found that there were no significant differences between the two
transducers. Actually the microchip transducer has a superior frequency
response, although this may not be clinically important for wave form
analysis.A variation of a ventricular catheter with a pressure measuring
transducer located at the tip of the catheter is the Ventcontrol MTC.17
This technology represents the direction in which ICP monitoring will evolve.
It is not easy to decide which system of ICP measurement is the best, because
of the large number of Variables, including cost. If access to the ventricle
is required, then a ventricular catheter and external transducer is both
cost-effective and reliable; it is the ‘gold standard’. However, most patients
now being monitored for ICP are likely to be suffering from head injury and
they will usually have narrow ventricles, making cannulation potentially
difficult for a young neurosurgeon. In the head-injury situation, the
preferred method is either a fiberoptic catheter-tip transducer (e.g. Camino
or InnerSpace) or an implantable transducer (e.g. Codman) inserted into brain
parenchyma, and this can be done at the bedside very simply. The choice
between these two types of transducer largely comes down to a question of
cost, which varies from country to country and is a individual decision.

Intraventricular pressure recording: The methodology
for measuring ICP has evolved progressively, with many workers preferring a
fluid coupled system using a ventricular catheter and an external transducer,
considering it to be the ‘gold standard’ of ICP measurement.

18
Ventricular ICP recording is the most reliable method in current use and it
has the advantage of minimal expense and maximal accuracy, since the external
transducer can be calibrated against an external reference at any time. The
equipment required is commonplace in any intensive care unit. The reference
point for an external transducer should be the foramen of Monro, because it is
close to the center of the head – 2 cm above the pterion, is a rough guide to
its surface marking. The midpoint of a line joining the two external auditory
meatus is another suitable reference point, although somewhat posterior to the
interventricular foramen. Other workers use the external auditory meatus.19
Whatever reference point is used, the level of an external transducer needs to
be altered with each change in head position. The ventricular method obviously
requires placement of a catheter into a lateral ventricle, and this may be a
technically difficult procedure because of a narrow or displaced ventricle.
Injury of the basal ganglia can occur with ill directed or over enthusiastic
attempts at ventricular cannulation. The infection rate is 3.6%, reaching a
potentially serious level after three days.3
Other quoted infection rates range from less than one percent to more than 5%.20
A big advantage of the ventricular method is that CSF can be withdrawn to
lower ICP. All joints in the recording system must be watertight. If they are
not, ‘micro-leaks’ will invalidate the pressure recordings. Each portion of
the system must be tested periodically by isolating the external system from
the patient temporarily and subjecting it to a pressure head of about 50mmHg.
After being isolated, the external system should maintain its intraluminal
pressure and if not, the connections must be tightened or the system discarded
and replaced with a watertight system.21
Sometimes, ventricular catheters
block and this can be overcome by flushing a small amount of sterile saline
through the system. However, repeated flushing should be avoided because of
the real risk of infection.

Other Methods: The hollow skull bolt (‘Richmond screw’)
has been widely used in many centers.

22
There have been many modifications to achieve a lower profile, CT
compatibility, more side holes (‘Leeds screw’), and a pediatric version.2,23-25
These devices are simple to insert but they have a tendency to block and so
produce a damped, inaccurate trace. At high pressures, the subdural bolt tends
to read lower than a ventricular catheter.26,27
This question of accuracy presents a major problem and is the main reason why
the hollow bolt method has declined in popularity.4
Subdural catheters can be useful where the ventricle cannot be Cannulated, but
they are also likely to underestimate the true ICP. The extradural site for
monitoring has been used and has the advantage avoiding penetration of the
dura. However, there are technical problems associated with the inelasticity
of the dura and the need for the transducer to lie flat (co-planar) on the
dura. Unfortunately, irregularities of the dura and inner table of the skull
are common. If co-planarity is not achieved, stresses and strains in the dura
can distort the measurements and falsely record high pressure.2,28
For these reasons concerning accuracy, the extradural method is now used very
infrequently.

Is ICP monitoring useful? The continuous measurement of
ICP is an essential modality in

most brain monitoring systems.
After a decade of enthusiasticattempts to introduce new
modalities for brain monitoring (tissueoxygenation, microdialysis,
cortical blood flow, transcranialDoppler ultrasonography, jugular
bulb oxygen saturation) itis increasing obvious that ICP is
robust, only moderatelyinvasive, and can be realistically
conducted in regional hospitals.Although there has been no
randomized controlled trial aboutinfluence of ICP monitoring on
overall outcome after followinghead injury, recent audit5
shows almost twofold lower mortalityin neurosurgical centres, where
ICP is usually monitored, versusgeneral intensive care units,
where it is not monitored. However,the availability of ICP
monitoring is not the only differencebetween neurosurgical and general
intensive care units thatmight explain the difference in
mortality after head injury. ICP waveform contains valuable information about
the natureof cerebrospinal pathophysiology.
Autoregulation of cerebralblood flow and compliance of
cerebrospinal system are both expressedin ICP. Methods of waveform
analysis are useful both to derivethis information and to guide the
management of patients.The value of ICP in acute states such as
head injury, poor gradesubarachnoid hemorrhage, and
intracerebral haematoma dependson a close link between
monitoring and therapy. CPP orientedprotocols,20,29
osmotherapy2
and the "Lund protocol"cannotbe conducted correctly without
ICP guidance. A decision aboutdecompressive craniectomy should
be supported by the close inspectionof the trend of ICP and,
preferably, by information derivedfrom its waveform.30
In encephalitis,31
acute liver failure24
and cerebral infarction after stroke,32
ICP monitoring is usedless commonly, however, an increasing
number of reports highlightits importance.A slightly different methodology
for CSF pressure interpretationis applied in chronic states such
as hydrocephalus or benignintracranial hypertension. In the first
case assessment of CSF,pressure–volume compensation and
circulation are essentialto optimize patient management.33
Volume-adding tests withparallel measurement of ICP and/or
overnight ICP monitoringwith waveform analysis have a special
role. In patients witha shunt in situ, who present with
persistent or recurring clinicalsymptoms, it helps to avoid
unnecessary shunt revisions.

This

is particularly important as patients
with a history of multipleshunt revision have a lower chance to
achieve good outcome inthe future. In benign intracranial
hypertension34
or craniostenosis35
ICP monitoring has been documented as useful both for diagnosisand to document response to
therapy.

Complications of ICP monitoring: The potential
complications of ICP monitoring include malposition, malfunction, infection
and hemorrhage. The incidence of each of these varies with the type of
monitoring being done and the experience of the personnel performing the
monitoring.

Malposition:

This is most commonly seen with intraventricular devices, where the catheter
either misses the ventricle or is inserted too far into the ventricle. The
subarachnoid bolt will under-read ICP if the dura is not properly opened, and
similarly all other devices have their own need for a correct technique of
insertion.

Malfunction: This is the common complication, occurring
in different ways for different types of monitors. If too much CSF is drained,
the ventricles collapse around the intraventricular catheters and they are
blocked. Parenchymal catheters had a major problem of drifting zero point and
they cannot be re-zeroed like the ventricular catheters, resulting in greater
inaccuracy with length of monitoring.

Infection: Infection in relation to ICP monitoring
generally refers to a positive culture of CSF or the device, and reported
infection rates vary widely and with the type of device. Intraventricular
device infections range from 0% to 10.5%. There is a general consensus that
the duration of monitoring has a direct relationship to incidence of
infections and that the infection rate climbs steeply after 5 days, though
this can be mitigated by subcutaneous tunneling of the device.

Hemorrhage: The limited published reports of hemorrhage
rate also vary, with an average incidence of 1.1% reported for
intraventricular devices. The incidence is approximately 2.8% for parenchymal
devices.

CONCLUSION

ICP monitoring is developed as a very useful tool,
Particularly in patients suffering from head injury. If a decision is made to
monitor ICP, then certain standards must be achieved so that reliance can be
placed on the data, which are obtained. Other physiological Variables such as
arterial BP are also recorded whenever possible. The catheter-tip and
implanted microchip transducers as having replaced the ventricular catheter
and external transducer as the ‘gold standard’ in ICP measurement. ICP
monitoring provides the only sure way of confirming or excluding intracranial
hypertension.

ICP monitoring provides the only reliable method of
assessing whether therapy will work and provide an early opportunity of
switching to an alternative therapy. If increased ICP is not present,
potentially dangerous treatment can be avoided. If the patient is paralyzed or
heavily sedated, conventional neurological observation is useless and ICP
monitoring provides a means of determining the patient’s cerebral perfusion
pressure (CPP) and an index of cerebral function.

23. Jackson H. The management of acute
cranial injuries by the early, exact determination of intracranial pressure,
and its relief by lumbar drainage. Surgery, Gynecology and Obstetrics
1922;34:494-508.